1. Field of the Invention
The present invention relates to power supplies for gaseous discharge lamps, and more particularly, to power supplies using at least two switched energy storage capacitors.
2. Description of the Prior Art
As illustrated in FIG. 1, prior art gaseous discharge lamp power supplies generally include a power supply which charges an energy storage capacitor to a comparatively high voltage, for example about 400 volts. Upon completion of the energy storage capacitor charging cycle, a trigger circuit generates a trigger voltage pulse having a narrow pulse width, but with a very high voltage, for example six thousand volts. The application of the trigger voltage to the lamp provides first stage ionization of the gas in the lamp and significantly lowers the internal impedance of the lamp from a nearly infinite value to a substantially lower value. The trigger voltage induced lamp impedance reduction enables the four hundred volt energy storage capacitor voltage to pass a current through the lamp which accomplishes second stage ionization of the lamp, further reducing the internal lamp impedance to a level on the order of one to two Ohms.
To achieve second stage ionization, the energy storage capacitor voltage must exceed a predetermined threshold voltage known as the minimum anode voltage at the time the trigger pulse is applied. As illustrated by FIG. 2, the magnitude of the minimum anode voltage is inversely proportional to the trigger voltage. As the trigger voltage increases, the minimum anode voltage decreases.
In FIG. 2, reference number 1 designates a hypothetical trigger voltage applied to the lamp to induce first stage ionization. Reference number 2 represents a specific energy storage capacitor voltage exceeding the lamp minimum anode voltage which will induce second stage ionization. If the energy storage capacitor voltage did not exceed the minimum anode voltage, second stage ionization of the lamp would not occur and the lamp would not generate useful output.
As the energy storage capacitor discharges current through the lamp, its voltage decreases below the minimum anode voltage, but the current flow through the lamp and its low impedance are maintained until the energy storage capacitor voltage decreases to the minimum holding voltage designated in FIG. 2 by reference number 3. Typical lamp minimum holding voltages vary about thirty to fifty volts. When the energy storage capacitor voltage falls below the minimum holding voltage, the gaseous interior of the lamp deionizes, the internal impedance of the lamp returns to its near infinite deionized state impedance and the output from the lamp terminates.
The cycle described above is repeated after the power supply recharges the energy storage capacitor to the desired peak capacitor voltage in excess of the lamp minimum anode voltage. A subsequent trigger pulse induces first stage ionization which reduces the internal impedance of the lamp and enables the fully charged energy storage capacitor to initiate second stage ionization.
In many applications, the gas discharge lamp depicted in FIG. 1 operates as a strobe light warning device for police car, fire engine, ambulance, aircraft and related optical signalling applications. In such applications, aluminum electrolytic capacitors are universally used as energy storage capacitors due to their low cost and extremely high power storage density. Electrolysis capacitors inherently possess a high equivalent series resistance ("ESR") which, due to the high output current levels required in strobe light applications, results in a significant I.sup.2 R capacitor power dissipation causing electrolytic capacitors used in strobe power supplies to operate at temperature approaching their design limits.
When operated at or near its temperature limit in a gaseous discharge circuit, the mean time before failure (MTBF) of an aluminum electrolytic capacitor is reduced to from about one hundred to one thousand hours, an extremely short operating lifetime for a manufacturer who may have to guarantee a strobe power supply for one year or for a user who desires a useful operating life of many years.
Although continuous operation does not represent a standard operating mode for a police or fire truck application, a strobe power supply intended for such applications was operated continuously until failure to evaluate the MTBF of a typical prior art strobe power supply circuit with its energy storage capacitor operating at peak temperature. In this particular test, the energy storage capacitor failed after three days or approximately seventy-two hours of continuous operation.
It is well known that the MTBF of an aluminum electrolytic capacitor decreases exponentially with increases in its operating temperature. For example, when operated at a comparatively low twenty-seven degree Centigrade temperature, an aluminum electrolytic capacitor may be expected to have an MTBF of tens of thousands of hours. When compared to the anticipated MTBF of one hundred to one thousand hours for a capacitor operating at its eighty-five degree Centigrade operating limit, the magnitude of the problem caused by the electrolytic energy storage capacitors becomes immediately apparent.
Although other types of capacitors such as metal film capacitors possess extremely low ESR ratings for the large capacitance values required in strobe power supplies, an equivalent metal film capacitor would be about four times larger than a comparable aluminum electrolytic capacitor and cost from four to ten times more. Utilizing low ESR energy storage capacitors such as metal film capacitors in strobe power supplies is therefore commercially impractical due to their large size and high cost.
Although the operating temperature of strobe power supply energy storage capacitors could be reduced by utilizing well designed heat sinks, the extra cost and volume requirements of heat sinks outweigh their potential benefits.
Strobe power supplies which generate a double flash output consisting of closely spaced primary and secondary flashes followed by a comparatively long time interval until the next double flash are currently very popular. In aircraft and vehicular applications, a double flash output is desirable because it is even more easily seen than a single strobe flash.
To generate the primary flash component of a double flash output, an energy storage capacitor must first be charged to a voltage higher than the strobe lamp minimum anode voltage and then be discharged through the lamp following application of the trigger voltage. Discharge of the energy storage capacitor during the primary flash reduces its voltage well below the minimum anode voltage. During the one tenth of a second time interval between the primary and secondary flashes, the power supply must recharge the energy storage capacitor to a voltage above the minimum anode voltage. In the energy storage capacitor voltage cannot be increased to a level above the minimum anode voltage during that one tenth of a second interval, the energy storage capacitor will not be able to produce the necessary second stage ionization in response to the second trigger voltage pulse and the secondary flash of the double flash group will not occur.
Prior art double flash strobe power supplies have solved this energy storage capacitor charging problem by reducing the capacitance of the energy storage capacitor and by increasing the rate that the capacitor charging voltage is applied from about four hundred volts per second to six hundred volts per second. These two changes enable the energy storage capacitor to be recharged to a voltage above the lamp minimum anode voltage during the one tenth of a second interval between the primary and secondary flashes of a double flash group.
Since the ESR of aluminum electrolytic capacitors increases as the capacitors rating decreases, the ESR of the lower capacitance energy storage capacitors used in double flash strobe power supplies is even higher than the already high ESR of capacitors used in single flash strobe power supplies. The higher voltage rating required for double flash energy storage capacitors increases the volumetric size and cost of these capacitors while the higher ESR ratings of these capacitors results in even higher capacitor operating temperatures. All of these factors taken together cause double flash strobe power supplies to be larger, less efficient and less reliable than comparable single flash strobe power supplies.